Summary [AniDB]: The story revolves around [Miyagawa Atsushi] and his sister [Miyagawa Atsumi]. Atsushi looks like an adult at 180 centimetres (about 71 inches) tall, but he is actually an elementary school boy. Atsumi looks like a little girl at 137 centimetres (54 inches) tall, but she is actually in high school.
Source: ANN